Error 815 W/B

Safety first: before doing anything switch the boiler off at the mains and isolate power at the fused spur. Do not work on gas components or inside the burner area unless you are a registered gas engineer. Wear eye protection and gloves. If you are unsure at any step stop and call a qualified engineer.

Initial homeowner checks you can do safely:

1) Try a reset: follow the boiler’s reset procedure (press and hold the reset/spanner as per manual for ~3 seconds) and see if the fault clears. If it returns, further checks are needed.

2) Visual inspection: with the boiler off and power isolated, remove any removable cover/panel (only if access is straightforward and you are comfortable doing so) and visually inspect the LLH sensor wiring and connector. Look for loose connectors, corrosion, chafed or broken wires, or signs the sensor lead has been pulled or damaged.

3) Check the sensor pocket: locate the low loss header and the sensor pocket. Confirm the sensor is fully inserted into its pocket and is in the correct location — some installations use a differential pair and their correct orientation/position matters. If the sensor is sitting outside the well or in the wrong port the reading will be wrong or absent.

Diagnostic steps (requires basic tools; isolate power first):

1) Check connector seating: unplug and re-seat the sensor connector. Inspect the pins for corrosion or bent pins. Reconnect firmly and restore power, then check the display for the fault repeating. If the connector was loose and the fault does not reappear you have likely fixed the issue; if it returns, continue troubleshooting.

2) Measure the sensor electrically (only if you are competent with a multimeter): isolate power and disconnect the sensor from the boiler wiring. Measure resistance across the sensor leads at room temperature. A functioning NTC sensor will show a finite resistance (not open circuit) and its resistance will change if you warm or cool the sensor slightly. If the sensor is open circuit (infinite resistance) or shorted (near zero ohms) it is faulty and needs replacement. Do not probe live terminals and do not attempt repairs on wiring you are not qualified to alter.

3) Trace the wiring: if the sensor looks OK, inspect the cable run between the sensor and the control unit. Look for damage, tight bends, or pinched sections. If you find damage, isolate power and either replace the cable or have an engineer do so.

4) Check for incorrect installation: some systems use a differential sensor assembly with two probes that must be fitted in specific flow/return pockets. If the probes are swapped or installed in the wrong pockets, the control can flag a fault. Verify the installation against the boiler/installer notes or call your installer to confirm correct placement.

If the above checks don’t resolve the fault:

- The sensor itself may need replacing. Replacing the sensor typically involves isolating the boiler, draining the relevant part of the heating circuit or isolating valves, removing the sensor from its pocket and fitting a correct replacement. This can require internal access and plumbing work; get a Gas Safe engineer to carry out the replacement and test the system.

- If a new sensor and intact wiring do not clear the fault, there may be a fault with the wiring to the control board, the connections on the control board, or the control module itself. Those components should be diagnosed and repaired by a qualified engineer.

After any repair: reset the boiler and observe operation. Check heating demand, pump operation and that the fault does not reappear. If the fault persists after sensible checks (connector reseated, sensor replaced, wiring checked), stop and call a Gas Safe registered engineer. They have the tools and training to safely replace sensors, perform electrical diagnostics, work on the hydraulic system (draining/refilling), and if necessary replace control modules. Do not attempt gas-valve, burner or sealed system repairs yourself. If you need help, contact a Gas Safe registered service engineer or the boiler manufacturer’s approved service partner.